Exhaust Flange

Four Pattern choices:

• 2-Bolt Round — the standard inline flange, with presets for 2.0, 2.25, 2.5, and 3.0 in bores.
• 3-Bolt Round — collector-style triangular pattern (bolts on a circle at 120° spacing), with 2.5, 3.0, and 3.5 in presets.
• 4-Bolt Oval — oval-port flanges with bolts at the rectangle corners, in 3.0, 3.5, and 4.0 in nominal presets.
• Custom — pick the bolt layout (2-bolt horizontal, 3-bolt circle, or 4-bolt rectangle) and bore shape (round or oval) independently, then dimension everything by hand.

The presets are fabrication references based on common aftermarket flange dimensions — not OEM-certified patterns. When you're matching a gasket or a measured flange, load the nearest preset and correct the fields, or go straight to Custom.

You never draw the outside profile. The generator places a circle around each bolt hole (bolt radius plus Bolt ear margin) and a boss around the bore (bore plus Bore boss margin), then computes the convex tangent envelope wrapping all of them — straight tangent runs blending into each ear and the boss, the way a properly made flange looks. The envelope is arc-fitted on output, so the curves import as real arcs. Outside smoothness controls the sampling density of that envelope, and Rotation spins the whole flange. Bore and bolt holes land on the HOLES layer by default, the outline on OUTER.

• The generator refuses bolt holes that crowd the bore — increase spacing or shrink the hole rather than fighting the error.
• Ear and boss margins are your meat-around-the-hole controls: bump the ear margin when you want more material under the bolt head, the boss margin for more weld shelf around the port.